Fix missing description placeholders in MQTT subentry flow (home-assistant#156684)

Author: jbouwh

homeassistant/components/mqtt/config_flow.py

@@ -4237,7 +4237,8 @@ async def async_step_entity(
         return self.async_show_form(
             step_id="entity",
             data_schema=data_schema,
-            description_placeholders={
+            description_placeholders=TRANSLATION_DESCRIPTION_PLACEHOLDERS
+            | {
                 "mqtt_device": device_name,
                 "entity_name_label": entity_name_label,
                 "platform_label": platform_label,

tests/components/mqtt/test_config_flow.py

@@ -3770,6 +3770,10 @@ async def test_subentry_configflow(
     assert result["type"] is FlowResultType.FORM
     assert result["step_id"] == "entity"
     assert result["errors"] == {}
+    assert "description_placeholders" in result
+    for placeholder, translation in TRANSLATION_DESCRIPTION_PLACEHOLDERS.items():
+        assert placeholder in result["description_placeholders"]
+        assert result["description_placeholders"][placeholder] == translation
 
     # Process entity flow (initial step)